A developing area of sulfur recovery technology is that of tail gas cleanup, that is, of removing trace quantities of sulfur compounds from gaseous effluent streams of Claus sulfur plants (Claus tail gas streams). Such gaseous effluent streams may contain substantial quantities of sulfur compounds. For example, gaseous effluent streams from a two-stage Claus or modified Claus plant typically can contain 3-10% of the sulfur present in the acid gas feed to the Claus plant in the form of elemental sulfur, hydrogen sulfide, sulfur dioxide, carbonyl sulfide, carbon disulfide, and the like.
Tail gas cleanup processes have been developed to further remove a large part of the residual sulfur compounds from the Claus tail gas streams to meet current environmental emissions requirements. Among the tail gas cleanup processes are those in which the amount of water vapor present in the gaseous effluent stream is reduced before further treatment. Examples include the Shell Claus Off-Gas Treating (SCOT) Process and the Beavon Sulfur Removal Process (BSRP). Such processes can reduce the water content of the Claus plant effluent stream, normally in the range of about 30-40 mol%, to less than, for example, about 5 mol% to facilitate removal of sulfur compounds from the tail gas. To remove the water, known processes may utilize, for example, reducing gas generators, hydrogenation reactors, and quench towers increasing the capital investment cost and operating cost for these and similar processes. Further, even after the bulk of sulfur compounds are removed from the Claus plant effluent streams by such tail gas treatment, there may remain residual hydrogen sulfide, for example, in the SCOT process, which must be converted to a suitable form for release to the atmosphere by, for example, incineration of residual sulfur compounds to sulfur dioxide (SO.sub.2). Such processes are reported capable of overall sulfur recoveries as high as 99.75%. However, to achieve such high levels of recovery, chemical (amine) absorption or oxidation, with attendant costs and operating and disposal problems must be used.
Removal of water from the gaseous stream after hydrogenation, while significantly facilitating the removal of hydrogen-sulfide, results in disadvantages associated with such water removal. First, of course, are the equipment costs required for water removal. Further, maintenance costs are significant because, for example, the mixture of hydrogen sulfide with water is a corrosive mixture which can require pH control and other methods to reduce corrosivity. Yet further, in the event of breakdown in the hydrogenation step, sulfur dioxide may be introduced along with the hydrogen sulfide into the water removal step forming an even more corrosive mixture. Greatly desirable therefore would be processes and systems capable of achieving a high level of recovery from Claus plant gaseous effluent streams which would eliminate the requirement of a water removal step.
Further, the use of, for example, chemical (amine) absorption or oxidation processes, such as for example, Beavon Stretford, to achieve extremely high overall sulfur recovery levels can entail high investment and energy costs, especially for regeneration, as well as expensive chemicals. Portions of these chemicals may be degraded and result in nonregenerable streams. Consequently, alternative processes which can ameliorate or eliminate some or all of these disadvantages are highly desirable.
To date, such processes have not been made available. Beavon, Canadian Pat. No. 916,897 (1972), for example, deals with using iron oxide for removal of residual sulfur from Claus plant tail gas to increase the overall recovery of sulfur to a level of 99%. Such level of recovery by itself, of course, is not sufficient to meet today's extremely low sulfur emission requirements. Courty, et al., U.S. Pat. No. 4,088,736 (1978), for example, deals with a zinc oxide based absorbent for use in removing hydrogen sulfide from industrial gases, and mentions as an example of such industrial gases, a Claus plant effluent stream. However, Courty, et al., apparently did not appreciate that Claus plant tail gas streams also contain sulfur dioxide which must be removed to meet emission requirements nor do Courty, et al., propose a precedure by which such removal can be accomplished.
It has been known for some years that metallic oxides such as iron oxide and zinc oxide are capable of absorbing hydrogen sulfide with the formation of sulfides in the purification of coal-derived gases. (See, for example, Britton, et al., U.S. Pat. No. 4,175,928 (1979), and Grindley and Steinfeld, "Development and Testing of Regenerable Hot Coal Gas Desulfurization Sorbents," Paper Presented at Second Annual Contractors Meeting on Contaminant Control in Hot Coal-Derived Gas Streams, Morgantown, W. Va., Feb. 17-19 (1982)). Nevertheless, similar processes have not been applied to the treatment of Claus plant tail gas streams as hereinafter described to achieve a Claus tail gas cleanup process capable of 99.97% overall sulfur recovery and higher while eliminating the need discussed above for chemical (amine) absorption or oxidation as required, for example, on the SCOT and BSRP processes, to obtain such levels of recovery, or to achieve a Claus tail gas cleanup process which can eliminate the need for a water removal and incineration steps and the attendant construction and operating problems discussed above.
Highly desirable are economic and effective Claus tail gas cleanup processes which do not require water removal and/or incineration and yet would be capable of meeting stringent air quality control requirements by providing recoveries as high as 99.97% or higher.